That thing can make a big difference when buying such a lightweight laptop. It probably weighs almost a pound. The laptop probably doesn't come with it for the same reason the iPad doesn't. This laptop is meant to be mobile, not docked somewhere most of the time.
Yeah, well. Some of the places at home where I use my MBA the most are too far from the outlet to be reached with a cable the length of the iPad's. The length of the MBA's cable is enough from the couch, but from outside, which is where I use it 75% of the time, I have to use the extension cable. I want to be able to use it when and where I want, not when and where Apple decides I should.
That's what mobile means. So yeah, I'm going to spend the freaking $19 on the freaking cable (which, I agree, is monstruous). That said, at least the USB-C charge cable is 2m, that's better than the MBA's, I think.